PSI - Issue 61

Enes Günay et al. / Procedia Structural Integrity 61 (2024) 34–41 E. Gu¨nay et al. / Structural Integrity Procedia 00 (2024) 000–000

39

6

0.45

1.2

)

)

2

2

0.4

1

0.35

0.3

0.8

0.25

0.2

0.6

0.15

0.4

0.1

g

=5

g

=5

0.05

d

d

0.2

g

=15

g

=15

d

d

0

g

=50

g

=50

d

d

Normalized Force in Lateral Direction (mN / m

Normalized Force in Normal Direction (mN / m

0

-0.05

0

0.25

0.5

0.75

1

1.25

1.5

1.75

0

0.25

0.5

0.75

1

1.25

1.5

1.75

Time (s)

Time (s)

(a) Normal Force

(b) Lateral Force

Fig. 3: Reaction forces on the indenter from polycrystal specimens with grain diameters of 5 µ m , 15 µ m , and50 µ m

0.4

0.35

0.3

0.25

0.2

0.15

0.1

Apparent Friction Coefficient

g

=5

d

g

=15

0.05

d

g

=50

d

0

1

1.25

1.5

1.75

Time (s)

Fig. 4: Calculated values of apparent friction coe ffi cient during scratching of polycrystal specimens with average grain diameters of 5 µ m , 15 µ m , and50 µ m

The results of the friction coe ffi cient calculation are given in Fig. 4, which shows that the friction coe ffi cient remains mostlyuna ff ected by the average grain diameter when the scratching process is displacement-controlled. The value of apparent friction coe ffi cient is approximately 0.37 in all cases throughout the simulations. Even while crossing grain boundaries, the value of friction coe ffi cient remains nearly constant, showing no significant changes. However, there is a slight increase in friction near the end of deformation for the g d = 50 µ m case. This points towards an increase in lateral force on the indenter that is not seen in other cases. To examine this matter in detail, the vertical displacement (i.e., in indentation direction) of nodes along the indenter path are plotted in Fig. 5 for all cases. This normalization is done by scaling the displacements to the smallest specimen’s size. Fig. 5a shows that all simulations have similar deformation patterns. Fig. 5b focuses on the region in front of the indenter, where it is apparent that there is more pile-up when g d = 50 µ m . The magnitude of the pile-up is 23% larger for the largest grain diameter case compared to the smallest. This is a result of weaker material response, which allows for more deformation, and hence, more pile-up, to occur. As the material pile-up increases at a higher rate when the grain diameters are larger, there is more material to deform, or scratch through, in the lateral direction. This leads to larger magnitudes of reaction force on the indenter in the lateral direction compared to normal direction, and thus an increase in the apparent friction coe ffi cient.